Get clients IP from the request packet.

ClientIpAddress.java

public final class ClientIpAddress {

  // CHECKSTYLE:OFF
  // http://stackoverflow.com/a/11327345/131929
  private static Pattern PRIVATE_ADDRESS_PATTERN = Pattern.compile(
      "(^127\\.)|(^192\\.168\\.)|(^10\\.)|(^172\\.1[6-9]\\.)|(^172\\.2[0-9]\\.)|(^172\\.3[0-1]\\.)|(^::1$)|(^[fF][cCdD])",
      Pattern.CANON_EQ);
  // CHECKSTYLE:ON

  private ClientIpAddress() {
  }

  /**
   * Extracts the "real" client IP address from the request. It analyzes request headers
   * {@code REMOTE_ADDR}, {@code X-Forwarded-For} as well as {@code Client-IP}. Optionally
   * private/local addresses can be filtered in which case an empty string is returned.
   *
   * @param request HTTP request
   * @param filterPrivateAddresses true if private/local addresses (see
   * https://en.wikipedia.org/wiki/Private_network#Private_IPv4_address_spaces and
   * https://en.wikipedia.org/wiki/Unique_local_address) should be filtered i.e. omitted
   * @return IP address or empty string
   */
  public static String getFrom(HttpServletRequest request, boolean filterPrivateAddresses) {
    String ip = request.getRemoteAddr();

    String headerClientIp = request.getHeader("Client-IP");
    String headerXForwardedFor = request.getHeader("X-Forwarded-For");
    if (StringUtils.isEmpty(ip) && StringUtils.isNotEmpty(headerClientIp)) {
      ip = headerClientIp;
    } else if (StringUtils.isNotEmpty(headerXForwardedFor)) {
      ip = headerXForwardedFor;
    }
    if (filterPrivateAddresses && isPrivateOrLocalAddress(ip)) {
      return StringUtils.EMPTY;
    } else {
      return ip;
    }
  }

  private static boolean isPrivateOrLocalAddress(String address) {
    Matcher regexMatcher = PRIVATE_ADDRESS_PATTERN.matcher(address);
    return regexMatcher.matches();
  }
}

GetIp.cls

String ipAddress = ApexPages.currentPage().getHeaders().get('X-Salesforce-SIP');

'True-Client-IP' - when the request is coming via the caching integration.
'X-Salesforce-SIP' - when the request is not via caching integration (sandbox, developer edition orgs) or via the secure url.

// GetUserIPAddress
public static String GetUserIPAddress() {
    string ReturnValue = '';

    ReturnValue = ApexPages.currentPage().getHeaders().get('True-Client-IP');

    if (ReturnValue == '') {
      ReturnValue = ApexPages.currentPage().getHeaders().get('X-Salesforce-SIP');
    } // get IP address when no caching (sandbox, dev, secure urls)

    system.debug('USER IP ADDRESS: ' + ReturnValue);

    return ReturnValue;
}